Computer simulation of the structure and properties of a particulate dispersion
A dispersion of 0.25p elongated iron particles has been simulated by a 3-D forcebias Monte-Carlo computation employing an ensemble of 1000 sphero-cylinders with aspect ratio 10 1. The particles exhibit a strong magnetostatic interaction, derived from a bulk magnetisation of 1700emu/cc, which is modelled as a pole-pole interaction. They also have a surface coating of surfactant which is modelled as a short range surface-surface repulsive potential. The energetic behaviour of the particle ensemble is determined by the interactions derived from these two potentials. A primitive magnetisation reversal algorithm is employed to lessen the effect of any artificially high energy interactions between particles, therefore investigations are limited to the cases of zero field and saturating field. An initial state of small particles is simulated by randon placements within a cubic cell with 3-D periodic boundary conditions. A secondary computation scheme is employed to expand slowly the particle lengths, including a corresponding scaling of the the magnetic moment. During the computation small groups of particles may become mutually bound by the strong magnetostatic interactions and exhibit co-operative behaviour. The simulation therefore includes a cluster-allocating algorithm and a cluster moving algorithm in order to take account of this behaviour. Analysis of the equilibrium configuration indicates that clusters, i.e. small groups of strongly bound particles, are an important characteristic of the dispersion microstructure. The interactions between clusters are predominantly negative in zero field and although they appear to be relatively weakly bound, they may give rise to extended networks of connected clusters. These considerations imply that the clustering of particles is a significant factor in determining the physical and magnetic properties of a dispersion.
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